Silver halide photothermographic imaging materials, especially "dry silver" compositions, processed with heat and without liquid development have been known in the art for many years. Such materials are a mixture of light insensitive silver salt of an organic acid (e.g., silver behenate), a minor amount of catalytic light sensitive silver halide, and a reducing agent for the silver source.
The light sensitive silver halide is in catalytic proximity to the light insensitive silver salt such that the latent image formed by the irradiation of the silver halide serves as a catalyst nucleus for the oxidation-reduction reaction of the organic silver salt with the reducing agent when heated above 80.degree. C. Such media are described in U.S. Pat. Nos. 3,457,075; 3,839,049; and 4,260,677. Toning agents can be incorporated to improve the color of the silver image of photothermographic emulsions as described in U.S. Pat. Nos. 3,846,136; 3,994,732 and 4,021,249. Various methods to produce dye images and multicolor images with photographic color couplers and leuco dyes are well known in the art as represented by U.S. Pat. Nos. 4,022,617; 3,531,286; 3,180,731; 3,761,270; 4,460,681; 4,883,747 and Research Disclosure 29963.
A common problem that exists with these photothermographic systems is the instability of the image following processing. The photoactive silver halide still present in the developed image may continue to catalyze print-out of metallic silver even during room light handling. Thus, there exists a need for stabilization of the unreacted silver halide with the addition of separate post-processing image stabilizers or stabilizer precursors to provide the desired post-processing stability. Most often these are sulfur containing compounds such as mercaptans, thiones, thioethers as described in Research disclosure 17029. U.S. Pat. No. 4,245,033 describes sulfur compounds of the mercapto-type that are development restrainers of photothermographic systems as do U.S. Pat. Nos. 4,837,141 and 4,451,561. Mesoionic 1,2,4-triazolium-3-thiolates as fixing agents and silver halide stabilizers are described in U.S. Pat. No. 4,378,424. Substituted 5-mercapto-1,2,4-triazoles such as 3-amino-5-benzothio-1,2,4-triazole as post-processing stabilizers are described in U.S. Pat. No. 4,128,557; 4,137,079; 4,138,265, and Research Disclosure 16977 and 16979.
Some of the problems with these stabilizers include thermal fogging during processing or losses in photographic sensitivity maximum density or contrast at stabilizer concentrations in which stabilization of the post-processed image can occur.
Stabilizer precursors have blocking or modifying groups that are usually cleaved during processing with heat and/or alkali. This provides the remaining moiety or primary active stabilizer to combine with the photoactive silver halide in the unexposed and undeveloped areas of the photographic material. For example, in the presence of a silver halide precursor in which the sulfur atom is blocked upon processing, the resulting silver mercaptide will be more stable than the silver halide to light, atmospheric and ambient conditions.
Various blocking techniques have been utilized in developing the stabilizer precursors. U.S. Pat. No. 3,615,617 describes acyl blocked photographically useful stabilizers. U.S. Pat. Nos. 3,674,478 and 3,993,661 describe hydroxyarylmethyl blocking groups. Benzylthio releasing groups are described in U.S. Pat. No. 3,698,898. Thiocarbonate blocking groups are described in U.S. Pat. No. 3,791,830, and thioether blocking groups in U.S. Pat. Nos. 4,335,200, 4,416,977, and 4,420,554. Photographically useful stabilizers which are blocked as urea or thiourea derivatives are described in U.S. Pat. No. 4,310,612. Blocked imidomethyl derivatives are described in U.S. Pat. No. 4,350,752, and imide or thioimide derivatives are described in U.S. Pat. No. 4,888,268. Removal of all of these aforementioned blocking groups from the photographically useful stabilizers is accomplished by an increase of pH during alkaline processing conditions of the exposed imaging material.
Other blocking groups which are thermally sensitive have also been utilized. These blocking groups are removed by heating the imaging material during processing. Photographically useful stabilizers blocked as thermally sensitive carbamate derivates are described in U.S. Pat. Nos. 3,844,797 and 4,144,072. These carbamate derivatives presumably regenerate the photographic stabilizer through loss of an isocyanate. Hydroxymethyl blocked photographic reagents which are unblocked through loss of formaldehyde during heating are described in U.S. Pat. No. 4,510,236. Development inhibitor releasing couplers releasing tetrazolylthio moieties are described in U.S. Pat. No. 3,700,457. Substituted benzylthio releasing groups are described in U.S. Pat. No. 4,678,735; and U.S. Pat. Nos. 4,351,896 and 4,404,390 utilize carboxybenzylthio blocking groups for mesoionic 1,2,4-triazolium-3-thiolates stabilizers. Photographic stabilizers which are blocked by a Michael-type addition to the carbon-carbon double bond of either acrylonitrile or alkyl acrylates are described in U.S. Pat. Nos. 4,009,029 and 4,511,644, respectively. Heating of these blocked derivatives causes unblocking by a retro-Michael reaction.
Various disadvantages attend these different blocking techniques. Highly basic solutions which are necessary to cause deblocking of the alkali sensitive blocked derivatives are corrosive and irritating to the skin. With the photographic stabilizers which are blocked with a heat removable group, it is often found that the liberated reagent or by-product, for example, acrylonitrile, can react with other components of the imaging construction and cause adverse effects.
Also, inadequate or premature release of the stabilizing moiety within the desired time during processing may occur.
Thus, there has been a continued need for improved post-processing stabilizers that do not fog or desensitize the photographic materials, and stabilizer precursors that release the stabilizing moiety at the appropriate time and do not have any detrimental effects on the photosensitive material or user of said material.